The effects of adequate total parenteral nutrition (TPN) on nitrogen excretion, urea N percentage, 3-methylhistidine excretion, and leg amino acid output, were studied during the ten-day period following abdominal surgery for generalized peritonitis in nine patients. The first two postoperative days were without nutritional intake, TPN was started on the third postoperative day (57 cal/KgBW--40% as Intralipid--0.30 g of N/KgBW). Leg amino acid outputs were done before TPN (DO), then two days (D2) and eight days (D8) after TPN. Total nitrogen and urea N percentage did not significantly differ before and after TPN. Between DO and D2 there was a significant reduction of urinary 3-methylhistidine (467 +/- 37 to 280 +/- 29 mumol/24 h-P less than 0.001) and leg amino acid release (604 +/- 103 to 254 +/- 87 nmol/mn/100 g of calf muscle--P less than 0.01) reflecting reduction in muscle hypercatabolism despite the persistence of the septic state. Between D2 and D8, 3-methylhistidine remained stable while leg amino acid release continued to decrease (254 +/- 87 to 68 +/- 40 nmol/mn/100 g--P less than 0.05). This association suggests an increased muscle protein synthesis. A closer examination of the clinical evolution of these patients, especially concerning their septic evolution, shows that only improved patients with recovery from sepsis increased their muscle protein synthesis. Thus, in septic hypercatabolic patients TPN seems to be able to reduce muscle catabolism while the increase in protein synthesis is mainly the consequence of recovery from the septic state. In such patients TPN should be used as a preventive therapeutic measure.